Actions can be adaptive or maladaptive. Thus, deciding which action to pursue often has a significant impact on health and well-being. Not surprisingly, therefore, the nervous system has evolved the means to evaluate actions, enhancing the adaptive ones and suppressing the maladaptive ones. But the neural mechanisms underlying action selection are poorly understood. The proposed research addresses this problem in the context of foraging behavior in the nematode C. elegans for two main reasons. First, this widely used model organism offers many unusual experimental advantages including a compact nervous system of only 302 neurons, a complete neuroanatomical wiring diagram, and a wide range of electrophysiological and optophysiological techniques for linking the activity of identified neurons to behaviors. Second, C. elegans foraging presents in simplified form one of the most fundamental issues in action selection, namely the question of how the nervous system associates the value of the outcome of an action with the particular behavior that caused it, without also reinforcing other behaviors at the same time. The findings from these studies are likely to accelerate the analysis of action selection in other organisms, including humans, by revealing simple neural circuits that will serve as road maps for more complex ones. PUBLIC HEALTH RELEVANCE: The long-term objective of the proposed research is to understand how the nervous system combines sensory information with internal knowledge of ongoing behavior to promote beneficial actions. This is a basic research question with significant implications for health because mental illness frequently manifests as a deficit in action selection and other forms of decision making. Approximately 65% of all human disease genes have a counterpart in C. elegans, including genes implicated in psychiatric conditions that effect decision making such as schizophrenia and bipolar disorder. Thus, findings from the proposed research are likely to provide insights into the molecular mechanisms of action selection.